Gas sensors, such as those used to determine the oxygen content of exhaust from automobile engines, are based on electrochemical cell measurements. Electrodes of such cells are often exposed to high temperatures and fast moving gaseous mixtures and the electrode on the outer surface of the cell suffers rapid deterioration. Mechanical baffles are generally used to reduce the deterioration, but it is often highly desirable to produce additional protection to extend the life of the sensor electrode.
As a protective measure for the outer electrode surface, porous oxide coatings have been proposed to cover the electrode, with such coatings applied by plasma or flame spraying techniques. Such techniques are comparatively expensive, however, and their application subjects the sensor and the electrode being coated to severe temperature shocks and the stress of high velocity, hot gas flow during the application of the coating. Such coatings are described, for example, in U.S. Pat. No. 3,645,875 which discloses application of a thin protective layer of solid refractory material over an electrode film, the protective layer being applied by firing a paste at a temperature of about 1750.degree. C or more, or by flame spraying which also requires high temperature treatment. Also, in U.S. Pat. No. 3,978,006, such a protective coating is disclosed which is formed primarily by brushing on an aqueous suspension of an oxide material and thereafter burning on, or sintering, at temperatures above the expected operating temperature of the sensor. Plasma spray or reactive vaporization which require such high temperatures are also suggested for application of the protective layer.
The present invention is directed to a process for forming a protective coating of oxide material over the electrode of a sensor element which is easier to effect and which does not subject the sensor and electrode to temperature or gas flow stresses or high firing temperatures.